The present invention relates to a process for preparing triphenylamine dimer, or triarylamine dimer useful as a material for electrophotography, a material for organic electroluminescence (EL), and the like. Particularly, the present invention concerns an industrially advantageous process for preparing triarylamine dimer, or triarylamine dimer utilizing the Ullmann reaction.
An electrophotographic photoreceptor is a member which forms a latent image of charges in a surface area when a beam light corresponding to an image has been irradiated. An organic electrophotographic photoreceptor has an organic photoconductive material on a conductive support. The organic photoconductive material is formed by binding photoconductive compounds with an organic resin.
In general, an organic photoreceptor has a charge-generating layer containing a charge-generating material which generates a charge by light irradiation, such as phthalocyanines, and a charge-transporting layer containing a charge-transporting material which transports a charge to the surface of the photoreceptor.
As a charge-transporting material used for an organic electrophotographic photoreceptor have been known triarylamine compounds, oxazole derivatives, oxadiazole derivatives, pyrazoline derivatives, hydrazone derivatives, hydrazine derivatives, triazine derivatives, quinazoline derivatives, styryl compounds, styryltriphenylamine compounds, butadiene compounds, carbazole compounds, etc. Benzofuran compounds recently reported (Japanese Patent Kokai Publication No. 179319/1997) may also be exemplified.
Triarylamine dimer has been used for an electrophotographic material, an organic electroluminescence (EL) material and the like. Particularly, it has been widely applied as a charge-transporting material (CTM) of organic electrophotographic photoreceptors used in copying machines, printers and the like or as a hole-transporting material (HTM) used in EL devices.
Hitherto, aromatic tertiary amine compounds having two aromatic tertiary amine and containing a biphenylene group in the molecule, such as triarylamine dimer, have been prepared by using benzidine as a starting material. Benzidine, however, is a carcinogenic compound, therefore, such a process is not put into practice today (See U.S. Pat. No. 3,484,467 and Japanese Patent Kokai Publication No. 321872/1994).
An example of a process for preparing triarylamine dimer without using benzidine as a starting material is disclosed in Japanese Patent Kokai Publication No. 17531/1998. Examples following Example 13 of that publication disclose a process for preparing triphenylamine dimer in which iodinated triphenylamine and hydroxyboronated triphenylamine are coupled in the presence of a palladium catalyst. This process, however, has drawbacks of that the expensive palladium catalyst is required, and separation and recovery of the catalyst after reaction is required.
On the other hand, a process utilizing the Ullmann reaction has been reported as a general process for synthesizing an aromatic tertiary amine compound. For instance, Synthesis, pp. 383 to 384 (1987) discloses a process for preparing triphenylamine using the reaction of the following scheme: ##STR2##
This reaction uses metallic copper as a catalyst, potassium carbonate as base, crown ether (18-crown-6-ether) as a reaction accelerator and o-dichlorobenzene as a reaction solvent.
EP0802173A1 discloses a process for synthesizing aromatic tertiary amines in which an aromatic halogenated compound and aromatic secondary amine are reacted in the presence of palladium tert-phosphine (for example, P(o-tolyl).sub.3 Pd) as a catalyst and tert-BuONa as base.
Alternatively, J. Org. Chem., Vol. 54, No. 6, 1989, pp. 1476 to 1479 discloses a process for synthesizing aromatic tertiary amines in which an aromatic halogenated compound and aromatic secondary amine are reacted in the presence of metallic copper as a catalyst, alkali metal hydroxide as base and polyethylene glycol (PEG) or polyethylene glycol dialkyl ether (PEGDM) as a reaction accelerator or a reaction solvent.
Although in the prior art on the Ullmann reaction, a variety of aromatic tertiary amines have been synthesized, no publication discloses an example of synthesizing triarylamine dimer.
Moreover, in the process using crown ether as a reaction accelerator, the crown ether is expensive and the process requires too much cost to be put into practice on an industrial scale. The process using palladium tert-phosphine has drawbacks of that an expensive palladium catalyst is required, and separation and recovery of a catalyst after reaction is required. The process using PEG or PEGDM as a reaction accelerator and a reaction solvent has a problem of that the yield of the product, which is no less than 40%, is too low to be carried out on an industrial scale.